The present invention relates to an electronic ballast for a discharge lamp having at least two lighting modes with different light outputs, and a lighting fixture using such ballast.
In a discharge lamp of a thermionic cathode type such as a fluorescent lamp, lamp illumination performance and a lamp life can be secured by maintaining an appropriate filament temperature in the lamp. FIG. 9 shows an example of typical lamp dimming control data defined according to the IEC. The horizontal axis employs numerical values obtained by dividing a lamp current Id by a reference current Itest. The vertical axis employs numerical values obtained by dividing the sum of squares of a large current side ILH and a small current side ILL of a lead wire current by the square of the reference current Itest, wherein a maximum dimming control curve A, a target dimming control curve B and a minimum dimming control curve C are defined. More specifically, a maximum target value, a recommended target value and a minimum target value are shown for a current Id flowing into the lamp filaments for each dimming level.
In the present specification, a lead wire current obtained during illumination is divided into a current referred to as a lead wire current which includes a lamp current (i.e. large current side), and a current referred to as a constant preheating current which flows via filaments (i.e. small current side). Indexes employed in the vertical axis in FIG. 9 are obtained by dividing the sum of squares of the lead wire current and the constant preheating current by the square of the lamp current, and can be said as indicating the conditions of a constant preheating current required for each lamp current.
A curve D in FIG. 9 (without any flow of a constant preheating current) is provided with numerical values plotted on an assumption that a lead wire current is equal to a lamp current and a constant preheating current is 0 [A]. If a lamp current which is substantially equal to a rating of a discharge lamp is made to flow (refer to an area in the vicinity of 1.0 in the lateral axis), the curve D substantially overlaps a target dimming control curve but shows indexes which decrease in accordance with reduction of a lighting output and fall under the lower target value in course of time (refer to an area less than 0.7 in the lateral axis).
That is, it can be said that a filament temperature suitable for illumination can be maintained in the vicinity of rated illumination of a discharge lamp by constantly preheating filaments using the lamp current. Dimming control, on the other hand, requires a larger constant preheating current to maintain the appropriate filament temperature in accordance with a lower lamp output.
As stated above, it is a well-know fact that a required amount of the constant preheating current is increased when dimming the discharge lamp.
An operation in an electronic ballast for a discharge lamp will be explained referring to a first conventional example shown in FIGS. 10 and 11 with respect to the present invention to achieve an appropriate flow of a constant preheating current during illumination by corresponding to a lighting output in a ballast having at least two lighting modes with different light outputs.
FIG. 10 is a circuit configuration of the discharge lamp ballast according to the first conventional example. FIG. 11 is a graph showing characteristics of a resonance circuit made of an inductor L2 and a capacitor C3 in each control state of preheating, starting, rated lighting and dimmed lighting, and a relationship between a constant preheating current flowing into filaments at that time and a driving frequency in an inverter 12.
A low frequency AC power source sent from a commercial power source 10 is rectified by a diode bridge including diodes D1 to D4 in a step-up chopper circuit 11. The voltage is stepped up by a step-up chopper circuit including a choke coil L1, a transistor Q1 and a diode D5. Obtained at both ends of an electrolytic capacitor C2 is a DC voltage of, for example, about 300V. This DC voltage is converted into a high frequency current in inverter 12 and used as a lighting current for a discharge lamp 13.
The inverter 12 has a half bridge inverter circuit including a pair of transistors Q2 and Q3, and an inverter control circuit 14 drives the transistors Q2 and Q3 to be turned on/off alternately to output high frequency power. The high frequency power is supplied to the discharge lamp 13 via filaments by passing through a DC blocking capacitor C4 and the inductor L2.
A control power source circuit 15 which includes a step-down chopper circuit or similar circuit generates a DC low voltage (e.g. 12V) which is supplied to the inverter control circuit 14 and a chopper control circuit 16. The chopper control circuit 16 may include a control IC (e.g. MC33262 made by Motorola, Inc.) and generates a gate control signal for the transistor Q1 in the step-up chopper circuit 11. The inverter control circuit 14 provides each gate of the transistors Q2 and Q3 in the inverter 12 with a signal oscillated by using a versatile control IC (e.g. μPC494 made by NEC cooperation) via a driver circuit (e.g. IR2111 made by International Rectifier Corp.).
When power is applied, the chopper control circuit 16 and the inverter control circuit 14 starts oscillation to bring an output voltage Vdc in the step-up chopper circuit 11 to about 300V and an oscillation frequency in the inverter 12 to fp=95 kHz. At this time, a voltage obtained at the filaments of the discharge lamp 13 is lower than a lamp starting voltage, which means the discharge lamp 13 does not light.
High frequency power outputted from the inverter 12 is also made to flow into a transformer T2 through a capacitor C9. Power induced to a secondary side of the transformer T2 causes a current to flow into the filaments of the discharge lamp 1 through capacitors C7 and C8. This current obtained before the discharge lamp 13 starts is a required preheating current of, for example, about 700 mA.
Preheating is carried out for two to three seconds and is followed by reducing an oscillation frequency in the inverter 12 to fs=80 kHz. As a result, the voltage at the lamp filaments is increased to a required starting voltage, whereby the lamp illuminates. Thereafter, the oscillation frequency in the inverter circuit 12 is reduced to fr=55 kHz to bring the discharge lamp 13 into a rated lighting state.
In the case where lamp dimming is desired (luminance which is lower than rated luminance), a dimming control signal is sent to the inverter control circuit 14. Therefore, the oscillation frequency in the inverter circuit 12 is changed to fd=75 kHz for the discharge lamp 13 to be in a dimmed lighting state.
FIG. 11 graphically shows the frequency characteristics of a voltage V1a applied to the discharge lamp 13, including (a) resonance characteristic at no loading when the discharge lamp 13 is turned off, (b) resonance characteristic in dimmed lighting, and (c) resonance characteristic at rated lighting. Impedance in the discharge lamp 13 is added to the resonance circuit in during lighting, so that the Q of the resonance circuit is reduced with a lower resonance frequency and resonance voltage than those obtained at no loading. FIG. 11 also shows a frequency characteristic of a filament current.
As dimming control is deepened with an increased oscillation frequency in the inverter circuit 12, the filament current is increased as understood from the filament current curve in FIG. 11. This is because of a resonance action in a resonant circuit made of the capacitor C9 and an inductance in a primary winding of the transformer T2. This resonant frequency is higher than a resonant frequency in the inverter in each control state of preheating, starting, rated lighting and dimmed lighting, which suggests that a current flowing into the filaments is larger in accordance with a higher operating frequency in the inverter, or a lower lamp voltage before the start of discharging or a lower lighting output during lighting.
It is therefore made possible to appropriately secure a required preheating current before the lamp starts and a constant preheating current during dimming control.
An operation of an electronic ballast will also be explained by referring to a second conventional example in FIGS. 12 and 13, with respect to an invention to lower power consumption by carrying out an operation to secure a preheating current in a preheating period and to prevent a constant preheating current from flowing after stable lighting.
FIG. 12 shows a configuration of an electronic ballast according to a second conventional example, including an AC power source 10, a rectifier DB for rectifying an output of the AC power source 10, a DC power source circuit 11a for smoothing an output of the rectifier DB. An inverter 12 converts a DC voltage outputted from the DC power source circuit 11a into high frequency power. A preheating circuit 5 includes a series circuit having a capacitor C9 connected across the output of inverter 12, a primary winding N1 of a preheating transformer T2 and a preheating switching element SW1. A control circuit 4 controls the preheating switching element SW1 to be turned on/off and the inverter 12. A load circuit 6 is made of a series circuit including a DC-blocking capacitor C4 connected between output ends of the inverter 12, a resonant inductor L2, and a discharge lamp 13 of a thermionic cathode type, and a resonant capacitor C3 connected in parallel with the discharge lamp 13. The two preheating windings N21 and N22 arranged in the preheating transformer T2 are connected to filaments F1 and F2 in the discharge lamp 13 via capacitors C7 and C8 respectively.
The control circuit 4 which controls the inverter circuit 12 and the preheating circuit 5 carries out each control of filament preheating, starting and lighting for the discharge lamp 13 after the AC power source 10 is supplied to start the inverter 12. The control circuit 4 includes: a timer circuit 41 for setting each switching time to switch operations in the inverter 12 from a preheating state to a starting state, and from the starting state to a lighting state, and switching time to switch operations in the preheating circuit 5 from a preheating current supplying state to a preheating current stopping state respectively, and outputting a control signal corresponding to each switching time. A frequency setting circuit 42 sets each operating frequency in the inverter 12 in the preheating state, the starting state and the lighting state in accordance with each control signal outputted from the timer circuit 41. A driving circuit 43 outputs a driving signal to determine switching of the switching elements in the inverter 12 on the basis of a frequency set by the frequency setting circuit 42. An inverter 44 outputs a control signal δ obtained by inverting a control signal Y which is outputted from the timer circuit 41 to control preheating switching element SW1.
An operation of the control circuit 4 will be explained below by using a timing chart shown in FIG. 13. First, after time point t0 to start driving the control circuit 4, the discharge lamp 13 is brought into the preheating state (preheating mode). An amount of time t1 to maintain the preheating state is set by a control signal α outputted by the timer circuit 41, during which the inverter 12 is subjected to a switching operation at a frequency fp set for the preheating state.
Next, after passing the time t1, the control signal α is switched from “L” to “H” for switching to a starting state (i.e. starting mode) to apply a voltage required for starting to both ends of the discharge lamp 13. An amount of time t2 to maintain the starting state is set by a control signal β outputted by the timer circuit 41, during which the inverter 12 is subjected to a switching operation at a frequency fs (fs<fp) set for the starting state.
Next, after passing the time t2, the control signal β is switched from “L” to “H” for switching to a lighting state (i.e. lighting mode) to supply power required for rated lighting of the discharge lamp 13. At the time t2 and thereafter, the control signal α is set to “H” and the control signal β is set to “H”, wherein the inverter circuit 12 is subjected to a switching operation at a frequency fr (fr<fs<fp) set for the lighting state at this time to realize lighting of the discharge lamp 13 by a predetermined output.
In the present conventional example, the control signal δ obtained by inverting the control signal Y which is switched from “L” to “H” at time t3 set as t1<t3<t2 is used to turn on the preheating switch element SW1 up to the time t3 so as to supply a preheating current, and the preheating switch element SW1 is turned off at the time t3 and thereafter to stop supplying a preheating current If.
More specifically, a preheating current is made to flow in filaments in the precedent preheating period and a constant preheating current supplied to the filaments is stopped after stable lighting. Therefore, power consumption by a constant preheating current which is unnecessary in normal lighting and adverse effects to a lamp life are prevented.